Aluminum borohydride complexes



United States Patent Ofiice 3,305,569 Patented Feb. 21, 1967 3,305,569 ALUMINUM BOROHYDRIDE COMPLEXES Ivan H. Skoog, East Oakdale Township, Washington County, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn. No Drawing. Filed July 26, 1961, Ser. No. 128,311 9 Claims. (Cl. 260448) This invention relates to novel high energy fuels for use in solid rocket propellant grains. More particularly this invention relates to nonvolatile non-hypergolic complexes of aluminum borohydride with polyethers which have been found to be suitable for use as high energy fuels in solid rocket propellant motor grains.

it is well known that the reactions of aluminum borohydride with suitable oxidizing agents are generally accompanied by relatively large energy releases and as a result there has been considerable interest in the use of aluminum borohydride as a fuel for use in rocket propellant compositions. Certain of its properties, however, render it impractical in solid propellant grains. Thus, for example, it is a liquid of relatively high vapor pressure (Le. melts at r 4.5 C., has a vapor pressure at C. of 119 millimeters and a boiling point of 445 C). and it is hypergolic in moist air. The fact that it is a mobile liquid renders its inclusion in a solid propellant grain, which is in effect a resinous or rubbery casting in which the fuel ordinarily serves as a binder for finely divided oxidizer, dilllcult or impossible. Furthermore, even it a solid propellant grain containing aluminum boronydride could be prepared, its vapor pressure would cause it to evaporate from the grain in a relatively short time. In addition, the hypergolic nature of the aluminum borohydride would render the preparation as well as the storage and handling of a grain containing it prohibitively hazardous. The ethyl ether complex of aluminum borohydride, which has also been considered for use as a fuel in solid propellant grains, has a very low vapor pressure at ordinary temperatures, but it is unfortunately still a liquid and is also hypergolic in the presence of moist air.

It is therefore an object of the present invention to provide novel complexes of aluminum borohydride with polyethers which are useful as high energy fuel com ponents of solid rocket propellant motor grains. It is another object of the present invention to provide a process for the preparation of these complexes. Other objects of the invention will become apparent to those skilled in the art from the specification which follows:

In accordance with the above and other objects of the present invention, it has been found that aluminum borohydride may be interreacted with polyethers having the formula:

wherein Q is selected from the class consisting of hydrogen, hydroxy, alkoxy and alkenyloxy, Q is selected from the class consisting of hydrogen, alkyl and alkenyl, R is selected from the class consisting of hydrogen, an alkyl group containing from 1 to 4 carbon atoms, an aryl group, an alkoxy group containing from 1 to 18 carbon atoms and an aryloxy group, W and Y are zero or positive integers the sum of which is from 0 to 5, X is 0 or 1, Z is zero when X is l and R is alkoxy or aryloxy, and Z is 1 otherwise and n is from about 50 to 100,000, provided that when Z is zero, the sum of W and Y is 1, to form nonvolatile, nonhypergolic aluminum borohydride-polyether complexes which vary in their physical characteristics from heavy, tacky greases to rubbery and resinous solid polymers. These novel complexes combine the high energy fuel characteristics of aluminum borohydride with handling and storage properties which render them suitable for use in solid propellant grains. It is noted that it is possible to prepare polyethers with end groups other than those specified and that such polyethers can be reacted with aluminum borohydride. Since the end groups intrinsically make up so small a part of the polyether, however, the substitution of one for another would have no substantial effect on the properties of the complexes prepared from the polyethers. The preparation of aluminum borohydride complexes of such polyethers is therefore considered to be within the scope of the present invention.

The rubbery complexes of the invention are suitable for use directly as binder-fuels for solid propellant grains; the non-rubbery complexes may be plasticize-d by conventional techniques until they are flexible and rubbery and the grease-like complexes can be utilized as plasticizers for other polymeric systems. Thus, for example, the grease-like complexes can be utilized to plasticize the non-rubbery, plastic complexes of the invention to form suitable binder-fuels for solid propellant grains. When these binder-fuels are mixed with oxidizing agents, such as nitrouium perchlorate, ammonium perchlorate, ammonium nitrate, sodium perchlorate, ammonium dichromate, urea nitrate, guanidine nitrate, triaminoguanidine perchlorate, etc, and ignited as by means of a heated wire, they burn with intense heat and the formation of large volumes of gases.

Among the polycthers which are suitable for use in the present invention are polymcthylene oxide, polymethyl iethylene oxide, polyethylene oxide, polypropylene oxide,

polytrimethylene oxide, polyphenylethylene oxide, polybutylmethylene oxide, polytetramethylene oxide, polypentamethylene oxide, polymethoxyethylene (i.e. polyvinyl methyl ether), polybutoxyethylene (i.e. polyvinyl butyl ether), polytetradecyloxyethylene, polyoctadecyloxyethylene, polyphenoxy ethylene, and copolymers thereof. Taking polymethylene oxide (frequently also called polyoxymcthylene) as exemplary, the following formulae (wherein n is as previously defined) illustrate various end groups of the polyethers:

HlCH Ol H uorcmoi u cH cHorcmoi H cu cuorcu os cn=cH nrcmos cn=cu notcn os cn The aluminum borohydride-polyether complexes of the invention are prepared by intermixing aluminum borohyride and a polyether of the type previously defined in an environment which is free of both oxygen and water vapor. The reaction is highly exothermic and care must be taken to maintain control thereof, eg by working with relatively small quantities, by adjusting the rate of mixing of the constituents, by cooling the reaction ves sel during the reaction, by diluting one or both reactants with organic solvents and thereby dissipating the exotherm, etc. Aromatic hydrocarbons and ethers such as dipropyl ether and diethyl ether are suitable diluents since they dissolve the aluminum borohydride and ordinarily the polyethers also. The particular polyether which is used also strongly affects the rate of the reaction and the exotherm.

The reaction is ordinarily carried out between about -20 C. and 100 C. The rate of reaction below -20 C. is very low or negligible while at temperatures above about 100 C. the control of the exotherm becomes more diflicult without attendant advantages over lower reaction temperatures. When the reaction is essentially complete (indicated by termination of the exotherm) the reaction product can be immediately isolated by simply removing the volatiles from the system, although such immediate isolation of the product is not necessary. It can be conveniently isolated at room temperature by removing the volatiles therefrom at reduced pressure.

Although the invention is in no way limited by this theory, it is believed that the complexes are formed by the sharing of electron pairs betwen aluminum atoms of the aluminum borohydride and ether oxygen atoms from the polyether. Whatever the reason, however, it appears that the complexes of the invention do not ordinarily contain more than about one molecule of aluminum borohydride per ether oxygen in the polyether. It is preferred that the molar ratio of aluminum borohydride to ether oxygen atoms in the complexes of the invention be at least about 0.1 since smaller relative amounts of aluminum borohydride tend to dilute the high energy properties of the complexes unnecessarily.

It is sometimes expedient to utilize a slight excess of aluminum borohydride over the relative amount desired in the final complex in order to compensate for the decomposition of a minor portion of the aluminum borohydride by traces of moisture or end groups in the polymer. This decomposition results in the evolution of small amounts of hydrogen and diborane and the addition to the residue of the corresponding aluminum. If the particular polyether which is used contains no end groups which are reactive with aluminum borohydride and care is taken to exclude all moisture, no allowance need be made for this reaction.

In order more clearly to disclose the nature of the present invention, several examples illustrating process and compositions thereof will now be described. It should be understood, however, that this is done solely by way of example of the best mode presently contemplated for carrying out the invention, and is intended neither to delineate the scope of the invention nor to limit the ambit of the appended claims. All parts are by weight unless otherwise designated.

benzene, toluene,

Example 1 About 0.143 gram (3.25 millimoles) of a polyethylene oxide which is a waxy solid having a molecular weight of about 4000, and which contains hydrogen, alkyl and hydroxyl end groups (available under the trade designation Carbowax 4000) is charged to an ampoule at room temperature. The ampoule is cooled to about l96 C. and 1.5 milliliters of benzene are allowed to distill into the ampoule from a connecting vessel which is at approximately room temperature. The contents of the ampoule are then warmed to about C. in order to ob tain solution of the polyether in the benzene. The ampoule is then cooled to -l96 C. and the pressure therein is reduced by connecting it to a high vacuum system. About 0.2324 gram (3.25 miliimoles) of aluminum borohydride (a liquid melting at 64.5 C., having a vapor pressure at 0 C. of 119 milliliters and a boiling point of 44.5 C.) is distilled into the ampoule from a vessel which is at approximately room temperature. The ampoule is then closed off by means of a stop cock, its contents are warmed gradually over a period of approximate 1y 2 hours to 40 C. and are allowed to stand at that temperature without stirring for several more hours. The volatiles are then removed by connecting the ampoule to a high vacuum system through series of traps which are held at -10 C., C., 1l0 C. and 196 C. Any gas passing the 196 C. trap is collected and measured. 0.2858 gram of a rubbery, tacky semi-solid aluminum borohydride-polyether complex remains in the ampoule after removal of all volatiles. This product is found to contain 4.27 percent of active hydrogen and 12.65 percent of aluminum and has a ratio of aluminum borohydride molecules to ether oxygens of approximately 1:4.8. The calculated contents of active hydrogen and aluminum for this ratio of aluminum borohydride to ether oxygen are 4.25 percent and 9.55 percent respectively. The active hydrogen analysis is performed by placing an analytical sample of the product in dioxane, slowly adding water and measuring the gas evolved with suitable correction (benzene can be used in place of the dioxane). The aluminum analysis is obtained from the resulting aqueous mixture. Boron analyses which are obtained in certain of the other examples herein are also taken from the aqueous mixture obtained from the active hydrogen analysis.

Details of the preparations of a number of complexes of aluminum borohydride with various polyethylene oxides and polypropylene oxides are provided in Table I. The manipulative steps in each lot are the same as those described above unles otherwise specified. The lot described above is repeated as Lot A in the table.

TABLE I Polyether Reaction Medium AltBI-h); Reaction Conditions and Lot v Amount lsolation 0! Product,

Amount Description Amount Desenptiou (grams) (grams) (grams) (1. 143 Polyethylene oxide 1 1. 5 Benzene 0. 2324 Combined at 1tl5; warmed,

pumped at 40 C. t). 107 Polyethylene oxide 9 Excess borohydride O. 2356 As in Lot A. 0.1234 .do 5 Toluene 0. 2152 Combined at warmed to 60, and pumped at 25. 0.188 c .do 10 do 0. 2162 Combined at 25; pumped at 25. 0. 1985 .do 5 Ethyl ether 0. 2481 Combined at -195; warmed,

pumped at 60. 0. 225 Block eopolymer of poly- 5 Benzene 0. 4202 As in Lot A.

ethylene oxide and Polypropylene oxide. 0. 205 Polypropylene oxide l 4 .do O. 3893 Do. 0. 335 Polypropylene oxide 4 3 Ethyl ether t). 4388 Do.

1 Curbownx 4000."

"A white granular solid softening at about 198 (3., having a molecular weight above 1.000.000, containing hydrogen, nlkyl and hydroxyl end groups and available under the trade designation Polyox Coagulant.

3 I n oil having a molecular eight of about 2,500, eontalning hydrogen, allryl and hydroxyl end groups and available under the trade designation, Pluronie L02."

4 A waxy solid melting at about 53-56 0., having a molecu lar weight of about 4000, containing hydrogen and hydroxyl end groups and available under the trade designation "Polyglycol 1 4000."

The characteristics of the products obtained from Lots A through H of Table I are summarized in Table II:

TABLE II Analyses Analyses Ratio of Calculated Lot Physical State Found, Ether to for this Percent Altlillm Ratio,

Percent A Ruhbery, Act. H, 4 27, 4. 8:1 Act. H, 4 25,

tacky, seml- A], 12.65 A1, 9.55 solid. 13 Gummy solid C Se ni-solid. ct. H, 5.47; 3. 4:1 Act. H, 5.44;

A], 13.9; B, 1, 12.2; 14.8. 14.7. D Resin Act. II, 3.72 E Gummy resin Act. H, 6.69; 2:1 Act. H, 7.52;

A1, 17.9; B, Al, 16.9; 19.65. B, 20. F Gummy resin-.. Act. H, 7.10; ea. 2 1

Al, .6; B, 20.2 G Solid Act. H, 6.59; 1.9:1 Act. H, 6.62;

A1,] .0; B, A1, 14.9; 18.4. B, 17.9. H Rubbery Act. H, 6.70; 1.65:1 Act. H, 7.18,

A1, 16.3; B, A1, 16.2; 19.3. B, 19.4.

Example 2 Table III summarizes the preparation of a number of additional polyether-aluminum borohydride complexes according to the invention in which polyethers other than polyethylene oxide and polypropylene oxide are utilized. The preparative steps are the same as Lot A of Example 1 except that the ampoules containing the reactants in the following lots are closed off at 196 C., the reactants are warmed gradually to 25 C. and are allowed to stand for several hours at that temperature before opening them and removing the volatiles therefrom.

TABLE III Polyethcr Reaction Medium A](BII1); Lot Amount Amount Description Amount Descrip- (grams) (grams) [111].) tion .I 0. 181 Polyoxymeth- 2 Toluene. 0. 172

ylene. K 0231 .....do 2 .(l0. 0.716 L. 0. 204 Polyvinyl 3 Benzene. 0.101

methyl ether. M... 0.139 Poly(trimeth- 2 .do 0.366

ylenc oxide). N 0.141 (Io None... 0.558

1 A white granular solid ground to pass through a 44 micron mesh screen having a molecular weight of about 32,000 (n about 1,000) which is available under the trade designation "1)elrin.

A stlfi resin having a molecular v eight of about 30,000-35,000 (it about 67) which is available under the trade designation Luthanol M40."

3 A white solid having a molecular weight 01' about 35,000 to 40,000 (it about 75).

In Lot K an excess of the aluminum borohydride is utilized and when the mixture is warmed to approximately C., a vigorous evolution of gas occurs. The reaction is found to be sufliciently violent to cause the loss of part of the di-borane and excess borohydride, although the temperature rises to only approximately 30 C. In

the reactions involving polyvinyh'nethyl ether and poly- (trimethylene oxide) (Lots L, M and N) insoluble solid products precipitate upon contact between the aluminum borohydride and the benzene solutions of the polyethers thereby resulting in somewhat heterogeneous products.

The amounts and natures of the products of Lots I through N are given in Table IV.

TABLE IV Analyses Analyses Ratio of Calculated Lot Physical State Found, Ether to for this Percent Al(BH4)-.t Ratio,

Percent Act. H, 2.67 J Tack 'solid 12-1 ii si i- 3 Act. H, 2.98; ,3 .4

B, 9.31. g; 2- Act. H, 7.43; K Viscous oll 3:1 1,215.13;

Act: H 6.39-

Act. H, 6.71 L Solid 1.8:1 Al,1..1;

Act. H, 4 14, B 181 B, 20.0. I 3 3 Act. H, 8.13 .1. M Act. 11,731, g

19.5. N do Act. 11,122, 9:! Act. II, 2.02

When polymethoxyethylene (Le. polyvinylmethyl ether) having hydrogen and vinyl end groups and a molecular weight of approximately 200,000, polyoctadecyloxyethylone having hydrogen and vinyl end groups and a molecular weight of approximately 75,000, polyphenoxyethylene having methyl and vinyl end groups and a molecular weight of approximately 10,000 and polyethylene oxide having hydrogen and vinyloxy end groups and a molecular weight of about 100,000 are each reacted with aluminum borohydride, according to the procedure of Lot A of Example 1, complexes according to the invention are formed which are useful as high energy fuels for solid propellant grains.

The use of the complexes of the invention as binderfuels in solid propellants requiring energetic fuels is illustrated as follows: About 329 parts by weight of ammonium perchlorate and parts of the complex of Lot A, Example 1 hereof are combined by milling. The resulting mass is molded under pressure into a propellant grain having the desired preselected shape.

This grain, when placed in a motor case and suitably bonded to the walls thereof according to the usual practice, can be ignited by means of a heated wire and pro duces a thrust of high specific impulse.

The terms and expressions which have been employed are used as terms of description and not of limitation, and it is not intended, in the use of such terms and expressions, to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

What is claimed is:

1. Complexes formed by reacting aluminum borohydrid-e with a polyether of the formula:

wherein Q is selected from the class consisting of hydrogen, hydroxy, alkoxy and alkenyloxy, 3 is selected from the class consisting of hydrogen, alkyl and alkenyl, R is selected from the class consisting of hydrogen, an alkyl group containing from 1 to 4 carbon atoms, an aryl group, an alkoxy group containing from 1 to 18 carbon atoms and an aryloxy group, W and Y are integers the sum of which is 0 to 5, X is 0 to 1, Z is zero when X is 1 and R is selected from the class consisting of alkoxy and aryloxy and Z is 1 otherwise and n is from about 50 to 100,000, provided that when Z is zero, the sum of W and 7 Y is 1 and that when X is 0, the sum of W and Y is not less than 1.

2. A complex according to claim 1 in which the molar ratio of aluminum borohydride to ether oxygens is from about 0.1 to about 1.

3. The proces which comprises intermixing aluminum borohydride and a polyether in an oxygen-free and water vapor-free environment at a temperature in the range of from about 20 C. to +100 C. for a time sufiicient to bring about significant production of an aluminum borohydride-polyether complex, the said polyether having the formula:

wherein Q is selected from the class consisting of. hydrogen, hydroxy, alkoxy and alkenyloxy, Q is selected from the class consisting of hydrogen, alkyl and alkenyl, R is selected from the class consisting of hydrogen, an alkyl group containing from 1 to 4 carbon atoms, an aryl group, an alkoxy group containing from 1 to 18 carbon atoms and an aryloxy group, W and Y are integers the sum of which is 0 to 5, X is O to 1, Z is zero when X is l and R is selected from the class consisting of alkoxy and aryloxy and Z is 1 otherwise and n is from about 50 to 100,000, provided that when Z is zero, the sum of W and Y is 1 and that when X is 0, the sum of W and Y is not less than 1.

4. The process of claim 3 in which the reactants are intermixed in the presence of an organic solvent selected from the class consisting of others and aromatic hydrocarbons.

5. A complex according to claim 1 wherein the polyether is polyethylene oxide.

6. A complex according to claim 1 wherein the polyether is polypropylene oxide.

7. A complex according to claim 1 wherein the polyether is polyoxymethylene.

8. A complex according to claim 1 ether is polyvinylrnethyl ether.

9. A complex according to claim 1 wherein the polyether is poly(trimethylene oxide).

wherein the poly- References Cited by the Examiner UNlTED STATES PATENTS 2,925,44l 2/1960 Brown 260--448 HELEN M. MCCARTHY, Acting Prinmry Examiner.

J. W. WHISLER, I. R. PELLMAN, H. M. S. SNEED,

Assistant Examiners. 

1. COMPLEXES FORMED BY REACTING ALUMINUM BOROHYDRIDE WITH A POLYETHER OF THE FORMULA:
 3. THE PROCESS WHICH COMPRISES INTERMIXING ALUMINUM BOROHYDRIDE AND A POLYETHER IN AN OXYGEN-FREE AND WATER VAPOR-FREE ENVIRONMENT AT A TEMPERATURE IN THE RANGE OF FROM ABOUT -20*C. TO +100*C. FOR A TIME SUFFICIENT TO BRING ABOUT SIGNIFICANT PRODUCTION OF AN ALUMINUM BOROHYDRIDE-POLYETHER COMPLEX, THE SAID POLYETHER HAVING THE FORMULA: 